1. Field of the Invention
The present invention relates to methods and apparatus for preventing power imbalance in a multiple input multiple output (MIMO) wireless preceding system.
2. Description of the Related Art
A new generation cellular mobile communication system has to provide wireless network applications with high-speed information transmission. Multiple Input Multiple Output (MIMO) schemes use multiple transmit antennas and multiple receive antennas to improve the capacity and reliability of a wireless communication channel. As disclosed in “Way Forward on 4-Tx Antenna Codebook for SU-MIMO”, 3GPP R1-072597, published by Texas Instruments, Motorola, Nokia, Ericsson, NTT DoCoMo, Panasonic, Freescale Semiconductor, Nortel, Huawei, Broadcom, Comsys, Marvell Semiconductor, Nokia Siemens Networks, InterDigital, Mitsubishi Electric, Sharp, “Way Forward on Codebook design for 4Tx SU MIMO”, 3GPP R1-072581, published by Samsung, ZTE, ETRI, Qualcomm, CHTTL, ITRI, Magnolia, Broadband, ASUSTec, Sunplus mMobile Inc., CGC, “Way Forward on SU MIMO Codebook design for 4Tx dual polarized antennas”, 3GPP R1-072582, published by Samsung, ZTE, ETRI, ITRI, ASUSTeK, CHTTL, Magnolia Broadband, Sunplus mMobile Inc., CGC, NTT DoCoMo, Mitsubishi Electric, single user MIMO (SU-MIMO) precoder has been designed to improve the spectral efficiency of a wireless system.
In all these prior designs, however, the constant modulus (CM) principle is not strictly followed. Constant modulus (CM) principle means the energy on each antenna is the same, for any given subcarrier in the whole OFDM band. Therefore, power imbalance does not exist if the constant modulus principle is met. As a result, a Node-B (i.e., base station) may experience power imbalance at the power amplifiers (PA) of different Node-B transmission antenna, which is detrimental to system performance. Although the Node-B scheduler may alleviate the power imbalance problem by grouping units of user equipment (UE) with different precoding spatial vectors and matrices in a way such that the total transmission power of the group is roughly balanced across different transmission antennas, the resulting loss of scheduling flexibility is undesirable from the Node-B operation and performance viewpoint.